Agriculture: Citrus Pest Management Guidelines

Armillaria Root Rot

  • Armillaria mellea
  • Symptoms and Signs

    Armillaria root rot, also known as oak root fungus, can occasionally damage and kill citrus trees. Symptoms may not develop until after the disease is well established. The first symptoms of Armillaria root rot are poor growth or dieback of shoots, small yellowing leaves, and premature leaf drop. The fungus spreads by root contact or through rhizomorphs (black strings of fungal mycelia), which can grow short distances through the soil and contact and penetrate citrus roots. The pathogen invades the roots and crown, eventually girdling the crown region and destroying the entire root system. From the infection site, the fungus invades lateral roots and the crown region, where it spreads as white mycelial plaques in the cambium region between the bark and wood. This distinguishes Armillaria from most other wood-rotting fungi, which grow in and decay the wood and not the cambium.

    In late fall and winter, Armillaria often forms clusters of mushrooms at the base of infected trees a few days after a rain.

    Comments on the Disease

    Armillaria root rot fungus is native to California's woody plants and affects many tree crops planted on hillsides, in former riverbeds, floodplains, and on other areas subject to overflow. The fungus can survive for many years in dead or living roots of fruit and nut trees, and on ornamental and native tree species. The disease often occurs along former streambeds and near creeks where the soil is moist and where dead roots or stumps harboring the fungus may be buried in the soil. The fungus requires cool, moist soil conditions for development and spread; it is therefore rarely a problem in desert areas.

    Management

    Management of Armillaria root rot relies primarily on preventing infection of new trees. Once infection is apparent, it is very difficult to save a tree. Avoid planting in a site likely to be infested with Armillaria. If there are infected trees in your orchard, remove them completely, including the roots, and let the trees dry thoroughly before disposing of them. Also remove the neighboring, apparently healthy trees; once symptoms appear on a tree, the disease has probably already spread to the roots of the surrounding trees.

    To prepare infested sites for replanting, remove stumps and roots of the diseased tree. Destroy roots larger than 0.5 to 1 inch (1.2–2.5 cm) in diameter and fumigate the site.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide’s properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    PREPLANT TREATMENTS
    A. CHLOROPICRIN*§
      (Chloropicrin 100) Label rates See label NA
      . . . PLUS . . .
      1,3 DICHLOROPROPENE*§
      (Telone II) Label rates See label NA
      COMMENTS: For Armillaria root rot management as a broadcast preplant fumigant. Strict usage regulations exist (see county agricultural commissioner). Apply as a double application: the first application is non-tarped Telone II (332 lb a.i./acre) plus chloropicrin (200 lb a.i./A) at 18 to 24 inches, followed by a second tarped (TIF) application of chloropicrin (150 lb/A) at 12 to 14 inches.
     
    B. CHLOROPICRIN*§
      (Chloropicrin 100) Label rates See label NA
      COMMENTS: For Armillaria root rot management at individual tree sites as a pre-plant fumigant. Use a probe and inject 0.5 to 1 lb of Chloropicrin 100 fumigant (CA Label) to a depth of 18 to 24 inches into the center of the area to be treated (must be 18 or more inches into the soil). Strict usage regulations exist (county agricultural commissioner).
    * Permit required from county agricultural commissioner for purchase or use.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    NA Not applicable.
    § Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.

    Important Links

    Text Updated: 01/19
    Treatment Table Updated: 01/19